In France, where medical abortion has been available longest, the majority of eligible women elect medical rather than surgical abortion when given a choice. Although not yet widely available in the United States, American women will soon also have a choice of several methods of medical abortion and may be expected to opt for medical methods. The goal of this project is to devise a standard method for analyzing the efficacy of medical abortion and to use the method to resolve several pressing clinical questions about the regimens. To date, research on the efficacy of medical abortion has followed the conceptual model used for analysis of surgical abortion; the failure rate is computed by simple division and expressed as a proportion. The problem, however, is that while surgical abortion is a discrete event occurring in the space of a few minutes or less, medical abortion is a process typically lasting from several days to several weeks. In this process, two events may occur that are not encountered with surgical abortion. A woman can opt out of the process before a determination of efficacy is possible by electing to have a surgical termination or a clinician can perform a surgical intervention that is not medically necessary. Protocols specify different maximum waiting periods. Probabilities of failure, therefore, should explicitly account for differences in observation periods. For these reasons, we propose and defend a conceptual scheme for analyzing efficacy with appropriate life-table procedures. We further propose to reanalyze data on more than 10,000 subjects in large multi-center clinical trials that have already been (or are currently being conducted) in the United States, Europe, and several developing countries. Following the scheme that we outline, we will produce new and consistent estimates of the efficacy of the main regimens and thereby help to resolve eight important unanswered clinical questions that will affect protocols for the regimens that will be adopted in the United States.